Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/256

Rh 238 RAILWAY action, are adopted on the main line ; and, as a general rule, switches on the main line are ordered to be laid with the points in the direction of the traffic, so that passing trains may run out of the points, and not into them. " Facing- points," as they are termed, are such as are laid on the main line, facing or pointing towards the regular advancing trains. Many accidents have been caused to trains by facing-jxrints, improperly set or out of order, turning the train unexpectedly into a siding, when it was impossible to pull up in time to prevent a collision, or throwing the train off the rails altogether. So dangerous are facing-points felt to be, particularly on high-speed lines, that on some railways they are absolutely forbidden at all except at terminal stations and at intermediate stations where every train is ordered to stop. In some situations this rule can only be followed by sacrificing simplicity and increas- ing the number of backing-points ; but it no doubt diminishes the risk of accident. Turn-tables are of two classes, for turning carriages and waggons, and for turning engines and tenders together. Those ordinarily used are of cast-iron, and carry two transverse lines of rails. They re- volve upon a central pivot and conical rollers near the circum- ference, which are upheld by and turn upon a cast - iron base bedded in ce- ment, or on a built foundation (see fig. 22). For turning engines and tenders to- gether turn- tables about 40 feet long are re- quired. A com- mon plan of table consists of two longitu- dinal balks of timber, to carry a line of rails, framed together with cast - iron beams in such a way that the FIG. 22. Carriage turn-table for stations, centre is supported on a pivot and the extremities on rollers. The table revolves, in a pit about 4 feet deep, on a large circular race of cast-iron bedded on a firm foundation to carry the rollers, and the motive force is applied by means of gearing. In situations where there is much traffic it is needful to extend the deck of the table laterally, like wings, to complete the circle, and so cover in the pit. Traversers are a convenient substitute for turn-tables, particularly for working a number of parallel lines of rails. A traverser is simply a low rectangular frame, made with two overhanging rails, to receive carriages or waggons, and movable on rollers across the lines of rail, so as to receive the carriage from any one line of rail and deposit it on any other. Water-cranes for delivering water to locomotives are too familiar to every one to need description. Railway Signals. The earliest passenger railways were opened without any fixed signals. Flags and disks, elevated on posts and pillars, were first employed, in various forms, and were worked on various codes. Sir Charles Hutton Gregory, about the year 1841, designed and erected at New Cross station, on the Croydon Rail- way, the semaphore signal, an adaptation of the old form of sema- phore used for telegraphing over short distances. This was the most important step ever taken in the development of railway signalling. The semaphore has been almost universally adopted for fixed signalling on railways. There are two arms, to the right and to the left, to command trains arriving in either direction. The arm is turned out horizontally, in a position perpendicular to the post, to signal danger ; diagonally downwards at an angle of about 45 3 as a signal of caution ; and it is turned home, disappearing within the post, when the line is right for the approach of a train. But the general practice now (1885) is to work the semaphore in two positions only, at danger and at caution. It is thus always in sight, and its position can be identified without hesitation. To make the signal system safe there must be clear definition and strict enforcement of the duties of the attendant ; good men must be selected at adequate pay, and they must have convenient, warm, well -fitted lodges, with ample window -space, within which they may keep a constant watch over the line without exposure to weather. At junctions and other important signal-stations the lodges should be raised some height above the surface, to give perfect supervision in every direction and prevent distraction. At night the place of semaphores or disks is supplied by large and powerful lamps with reflectors, capable of showing lights of three colours, a white light, a blue or green light, and a red light, signi- fying respectively safety, caution, danger ; or, as in general prarti< v, two lights only are shown, red and green. Signalling has been a subject of much controversy, and has been divided into two main systems. In " negative " signalling the normal position is that of caution, or that of safety, as the practice may be, and the signal is only turned on to danger when specially required for the protection of the station on the line. On the other hand, the " positive " system presupposes the normal state of the signal to be that of danger, so that, if the signalman neglect his duty to lower the semaphore when the station is clear for the passage of an approaching train, the train is bound to stop. The positive system has long been in successful operation at all largo and important junctions., In such a situation the use of a stringent code is manifestly conducive to the greatest degree of safety, as by the unavoidable intersections of the lines of rails there are many chances of collision. The positive system has been merged in what is known as the block system of signalling, that is to say, Block the positive system has been on most railways extended to every system station on the lines in combination with telegraphic signalling. The best, perhaps the only, safeguard against error on the part of the persons in charge of trains is to be found in the adoption of the absolute block system, and of means for enabling engine-drivers to observe signals well in advance. The absolute block system con- sists in dividing the line of railway into intervals of convenient lengths, and by means of telegraphic and fixed signals allowing only one train at a time on any single length of single way. The signalman at station A does not send a second train to station 13 until he receives a signal from station 13 that the first train has arrived there ; meantime the signal at A stands at danger until the man at B signals the arrival of the train at B. Under the " per- missive block " system it is simply permitted to signalman B to block signalman A in the event of anything occurring at station B that may render that course advisable. But, supposing that a train has just left station A, then the message from B comes too late to enable signalman A to prevent the train from running into the obstruction at B. The permissive system has been well tried on the principal railways, and is preferred on some lines because it admits of trains being passed on one after another with greater rapidity than on the absolute block system. But it does not afford much protection, and it is now generally preferred to work on the absolute block system, and, for the purpose of doing so effectually, to erect intermediate stations on lines of constant traffic, so as to provide shorter intervals for blockings and obviate the delay inci- dental to unduly long intervals. The average distance apart of passenger stations is, say, 3 miles, but the distance of signal-sta- tions, whilst it seldom exceeds 4 miles, is frequently only a quarter of a mile, and the average interval may be taken as 1| miles. Dis- tant signals that is, signals placed at a distance in advance of points of danger and worked by wire communication from the signal-box were, it is believed, first introduced on the North British Railway at Meadowbank station near Edinburgh in 1846, after the opening of the Hawick branch. In 1852 the Great Northern Railway was completely fitted with distant signals of the semaphore type. Distant signals are occasionally fixed at 1500 yards' distance ; but beyond 800 yards their action is uncertain, and it is checked by a repeater electric or mechanical by which, by way of confirmation, the signal is returned to the signalman. As railway junctions were multiplied it became apparent, not only that distant signals were to be provided for distinct lines, but that concerted action should be established between signals and switches. They are said to be connected when they are simply coupled together and are moved simultaneously. They are said Inter- to be interlocked when the necessary movement of the switches is lockin completed before that of the signal to safety is commenced ; and, systen conversely, the movement of the signal to danger is completed before the movement of the switches can be commenced. This is the fundamental principle of the interlocking system of signalling now generally practised. By the combination of the absolute block system and the interlocking system the greatest possible number of trains are enabled to travel over one pair of rails in a given time. At Cannon Street station, at the busiest time of the day, eighteen trains arrive and eighteen depart within an hour; 108 operations of shifting switches and signals, by means of sixty-seven levers or handles, have to be performed in that time. On the North London Railway, at Liverpool Street station, 250 trains pass over the same rails in a day of nineteen hours, averaging only four minutes between trains ; frequently only two minutes elapse. The number of trains daily using Moorgate Street station on the Metro- politan Railway is more than 770, involving twice as many move- ments of engines 1540 movements on four lines of way in nine- teen hours, and every movement is separately signalled. This, of course, could not be performed without the aid of electric iustru-